Vibrating meters such as, for example, densitometers, volumetric flow meters, and Coriolis flow meters are used for measuring one or more characteristics of substances, such as, for example, density, mass flow rate, volume flow rate, totalized mass flow, temperature, and other information. Vibrating meters include one or more conduits, which may have a variety of shapes, such as, for example, straight, U-shaped, or irregular configurations.
The one or more conduits have a set of natural vibration modes, including, for example, simple bending, torsional, radial, and coupled modes. The one or more conduits are vibrated by at least one driver at a resonance frequency in one of these modes, hereinafter referred to as the drive mode, for purposes of determining a characteristic of the substance. One or more meter electronics transmit a sinusoidal driver signal to the at least one driver, which is typically a magnet/coil combination, with the magnet typically being affixed to the conduit and the coil being affixed to a mounting structure or to another conduit. The driver signal causes the driver to vibrate the one or more conduits at the drive frequency in the drive mode. For example, the driver signal may be a periodic electrical current transmitted to the coil.
One or more pick-offs detect the motion of the conduit(s) and generate a pick-off signal representative of the motion of the vibrating conduit(s). The pick-off is typically a magnet/coil combination, with the magnet typically being affixed to one conduit and the coil being affixed to a mounting structure or to another conduit. The pick-off signal is transmitted to the one or more electronics; and according to well-known principles, the pick-off signal may be used by the one or more electronics to determine a characteristic of the substance or to adjust the driver signal, if necessary.
Typically, in addition to the conduits, vibrating meters are also provided with one or more meter components, such as a case, a base, flanges, etc. While essentially all of the additional meter components can create measurement problems due to various vibrational characteristics, the vibrational characteristics of the case are typically most prevalent and cause the most significant measurement problems. Therefore, although the case is the focus of the discussion that follows, similar vibrational problems and solutions are applicable to other meter components. The measurement problems caused by various meter components is due to the difficulty in differentiating vibrations associated with the conduits from vibrations associated with the meter component, such as the case. One reason for the difficulty is that similar to the conduits, the case also has one or more natural modes of vibration, including for example, simple bending, torsional, radial, and lateral modes. The particular frequency that induces a mode of vibration generally depends on a number of factors such as the material used to form the case, the thickness of the case, temperature, pressure, etc. Vibrational forces generated by the driver or from other sources in the material processing system, such as pumps, may cause the case to vibrate in one of the natural modes. It is difficult to generate an accurate measurement of a characteristic of the substance in situations where the frequency used to drive the one or more conduits in the drive mode corresponds to a frequency that causes the case to vibrate in one of its natural modes of vibration. The vibrational modes of the case can interfere with the vibration of the conduits leading to erroneous measurements.
There have been numerous prior art attempts to separate the frequencies that induce the case's vibrational mode from the conduits' vibrational mode. These frequencies may comprise the natural resonance frequencies of the various vibrational modes of the case and the fluid filled conduits. For example, the case can be made extremely stiff and/or massive in order to decrease the frequencies that induce the various vibrational modes away from the anticipated drive mode of the conduits. Both of these options have serious drawbacks. Increasing the mass and/or stiffness of the case results in complex and difficult manufacturing, this adds cost and makes mounting the vibrating meter difficult. One specific prior art approach to increasing the mass of the case has been to weld metal weights to an existing case. This approach does not adequately dissipate vibrational energy in order to reduce the case's resonant frequencies. Further, this approach is often costly and produces an unsightly case.
One reason for the overlap between the drive frequency and the frequencies that induce a vibrational mode in the case is that the conduits and the case are typically formed from similar materials, i.e., both are formed from metal. While metal cases provide a number of advantages such as increased strength, explosion proof ratings, etc., metal cases add a significant cost to manufacturing a vibrating meter. A significant cost associated with the metal case is due to the required welding of the case. Additionally, a significant amount of time and/or cost is spent on adequately separating the frequencies that induce modes of vibration in the case from the drive frequency. The added mass or thickness of the case requires not only additional material but also additional time to assemble. Therefore, the use of a metal case with metal conduits has a number of drawbacks.
The present invention overcomes these and other problems and an advance in the art is achieved. The present invention provides a vibrating meter with an improved meter case. The meter case is formed from a high-damping material. The resonant frequencies of the meter case are reduced and separated away from the resonant frequencies of the conduits. Consequently, the risk of the drive mode of the vibrating meter inducing a mode of vibration in the meter case is substantially reduced. Furthermore, the cost associated with weld joints is substantially eliminated by the meter case of the present invention.